In optical fiber communications, connectors for joining fiber segments at their ends, or for connecting optical fiber cables to active or passive devices, are an essential component of virtually any optical fiber system. The connector or connectors, in joining fiber ends, for example, has, as its primary function, the maintenance of the ends in a butting relationship such that the core of one of the fibers is axially aligned with the core of the other fiber so as to maximize light transmissions from one fiber to the other. Another goal is to minimize back reflections. Such alignment is extremely difficult to achieve, which is understandable when it is recognized that the mode field diameter of, for example, a singlemode fiber is approximately nine (9) microns (0.009 mm). Good alignment (low insertion loss) of the fiber ends is a function of the alignment, the width of the gap (if any) between the fiber ends, and the surface condition of the fiber ends, all of which, in turn, are inherent in the particular connector design. The connector must also provide stability and junction protection and thus it must minimize thermal and mechanical movement effects. These same considerations apply to arrangements where the fiber, terminated in a plug connector, is to be used with active or passive devices, such as, for example, computers or transceivers and the like.
In the present day state of the art, there are numerous, different, connector designs in use for achieving low insertion loss and stability. In most of these designs, a pair of ferrules (one in each connector or one in the connector and one in the apparatus or device), each containing an optical fiber end, are butted together end to end and light travels across the junction. Zero insertion loss requires that the fibers in the ferrules be exactly aligned, a condition that, given the necessity of manufacturing tolerances and cost considerations, is virtually impossible to achieve, except by fortuitous accident. As a consequence, most connectors are designed to achieve a useful, preferably predictable, degree of alignment, some misalignment being acceptable. Alternatively, a device meant to accept a connector with the ferrule aligned in a bore and which comes to rest at a stop in the optical plane is acceptable.
Alignment variations between a pair of connectors can be the result of the offset of the fiber core centerline from the ferrule centerline. This offset, which generally varies from connector to connector, is known as "eccentricity", and is defined as the distance between the longitudinal centroidal axis of the ferrule at the end face thereof and the centroidal axis of the optical fiber core held within the ferrule passage. The resultant eccentricity vector has two components, magnitude and direction. Where two connectors are interconnected, rotation of one of them will, where eccentricity is present, change the relative position of the fibers, with a consequent increase or decrease in the insertion loss of the connections. Where the magnitude of the eccentricities are approximately equal the direction component is governing, and relative rotation of the connectors until alignment is achieved will produce maximum coupling.
In U.S. patent application Ser. No. 09/363,908, of Andrews et al., filed Jul. 28, 1999, now pending there is shown an arrangement for "tuning" a connector to achieve optimum direction of its eccentricity.
Unfortunately, there are a number of other problems affecting insertion loss of the plug connection, particularly where the connector plug, which terminates, for example, a jumper cable, is used to connect through a backplane to, generally, a piece of equipment having a connector adapter or connector receiving means mounted thereon. By "backplane" is meant, generally, a wall which separates internal apparatus from external apparatus, and through which a connection or connections are made. Thus the interior apparatus may comprise printed wiring boards (PWBs) having connector adapters mounted thereon (a circuit pack) which mate with plug connectors, such as LC type connectors which are mounted in the backplane. The backplane may also comprise a mounting panel as in a switch box, with the apparatus on one side thereof and the plug connectors insertable from the other side. In general use, the floating connector is affixed to the backplane, and the circuit pack is plugged into the backplane as needed to mate with the backplane connector plug.
In all such arrangements, manufacturing tolerances can add up to serious misalignments in any of the X, Y, or Z axes. Thus when a coupling adapter or device receptacle with a circuit pack is mounted on a PWB, the PWB mount, the adapter mount, the adapter itself and the latching mechanism of the circuit pack, which have all been made to be within certain tolerance limits, could, for example, all be at the extreme tolerance limits, thus presenting a particular misalignment of the adapter connector ferrule receptacle along one or more of the X, Y, Z axes. When an adapter is inserted into the wall of the backplane, it may be seriously misaligned with the ferrule which is latched to a receptacle on the backplane. In many instances the insertion of the adapter into the plug is blind, i.e., the operator cannot see one or the other coupling components, and the operator cannot easily feel for the correct position. This results in damage to the ferrule of the plug. As a consequence, insertion loss may be increased to an undesirable level. In extreme cases, connection might not be possible. In addition, severe improper Z axis travel of the ferrule can result in twisting of the ferrule barrel of the connector, resulting in de-tuning of the plug connector when it has been tuned.
In addition, in a backplane wall in the Z axis, a standard LC plug connector may not be long enough to insure that the spacing between the optical plane, where the backplane plug ferrule abuts the device stop or ferrule assembly in the abut, and a reference face in the front of the backplane remains the same regardless of the backplane thickness. If it does not, proper abutment of the two ferrules may not occur, thereby causing a material increase in insertion loss.